Coking plant with flue gas recirculation

ABSTRACT

Improvement in carbonization in a carbonization furnace and simultaneous reduction in NO x  emissions is achieved by recirculation of waste gas from a coking oven back to the oven chamber, the downcomers, or the sole channel system.

The invention relates to a carbonization plant designed and built according to the Non-Recovery Process or Heat Recovery Process for the production of coke from coal. A high throughput rate is particularly important to achieve economic efficiency of a carbonization plant according to the Non-Recovery Process or Heat Recovery Process, hereinafter briefly referred to as NR/HR. It is primarily due to the fact that a prolonged operating time, i.e. less economic efficiency, is always to be assumed for this technology since compared with the conventional horizontal chamber technology the release of combustion gas can only be slightly influenced. The velocity of this carbonization technology can only be influenced by an even supply of air to the process at several stages to optimize combustion.

In the past years, a great deal of improvements had therefore been proposed to homogenize the feed of primary and secondary air in the upper and lower oven in order to ensure a planar heating of the coal/coke charge from top to bottom. It is thereby possible to shorten the operating time required for a complete carbonization of the coal charge and to increase economic efficiency. Nevertheless, present solutions just represent an approximation to a planar heating because primary air in the upper oven and secondary air in the lower oven can always be supplied only spot-wise via the oven ground area.

An example for the refractory build-up in the lower oven is presented in the top view shown in FIG. 1. The crude gas/waste gas mixture formed in the combustion chamber of the upper oven is supplied to the sole flues in the lower oven in 2 to 20 downcomer channels per oven. There it is completely burnt by addition of combustion air. The heat generated there serves for carbonization of the coal charge from the bottom, thus ensuring a shortened operating time and a high performance rate of the oven. To this effect, so-called secondary air is sucked through openings at the front side in the lower oven and rendered available via a ramified vertical channel system to the actual sole channel heating flues for secondary combustion of combustible gases. During this process, a multitude of short individual flames is created in the sole channels. The heat generated in these sole channel heating flues is then vertically supplied via heat conduction through the oven sole of the coal charge for carbonization of this coal charge. The illustration clearly shows that the multiple-channel setup of the lower oven hardly offers any possibility for increasing the number of secondary air stages and thus for raising the efficiency of secondary combustion. Such a solution would also entail an unreasonably high extra expenditure on calibration procedures in terms of process technology.

Moreover, in the sense of an environmentally friendly oven operation, it is required to reduce nitric oxide (NO_(x)) emissions from an industrial plant to the greatest possible extent. Nitric oxides occur in processes of combustion of fossil fuels, e.g. coal, in the flame and in the surrounding high-temperature zone by a partial oxidation of the molecular nitrogen of combustion air as well as of the nitrogen bound chemically in the fuel. Thermally formed NO as main NO_(x) constituent develops from molecular nitrogen N₂ in the flame by oxidation with molecular oxygen at temperatures>1300° C. Since temperatures of up to approx. 1450° C. may occur in a NR/HR oven, technical efforts are to be taken to reduce this thermal NO formation and thus the resultant ecological burden. The most significant theoretical possibilities for NO reduction are comprehensively outlined in the following illustration:

-   -   low air figure in total     -   arrangement of air stages     -   NH3 injection     -   steam/water injection     -   waste gas recirculation.

To solve these two sets of problems outlined hereinabove efficiently and jointly, it is proposed to apply the process engineering measure of waste gas recirculation in the combustion chambers of the NR/HR oven. On the one hand, an internal waste gas recirculation in the sole channel system of the lower oven can be applied. Accordingly, a partial waste gas stream is branched-off immediately prior to its final evacuation from the oven in the sole channel and returned via a channel system or via one or several aperture(s) upstream into the sole channel. The drive for the waste gas recirculation is given by the pressure difference between the sole channels located upstream and downstream which causes a recirculation into the channel located upstream. The pressure difference is attributable to the higher waste gas temperature and thus to the lower density in the sole channel located upstream.

${\Delta \; p_{{{Index}\; 2} - {{Index}\; 1}}} = {g*\left( {\frac{{\overset{\_}{p}}_{2}}{{\overset{\_}{T}}_{2}} - \frac{{\overset{\_}{p}}_{1}}{{\overset{\_}{T}}_{1}}} \right)}$

This measure causes retardation in secondary combustion, it prolongs the individual flames in the sole flue and it promotes homogenization of the burn-off characteristics as well as the release of heat in the lower oven. Moreover, by way of this measure, the oxygen partial pressure in the sole channel heating flues of the lower oven is decreased, which results in a reduction of the thermally formed NO_(x) waste gas portion. The reason is that due to the admixture of waste gas the temperature of media and thus the thermal NO formation in the sole channel is reduced.

However, it is also possible to withdraw the waste gas only in the further run of the flow, i.e. externally from the channel system of the oven and to return it via a blower of the oven chamber to the downcomers or to the sole channel system in the lower oven. In an intermediate process technology treatment stage, further constituents affecting the environment or process can be deprived from the waste gas before they are returned into the oven.

The invention solves this task by means of the characteristic features designated in the claims. It is further elucidated in the drawings FIG. 1 to FIG. 5.

FIG. 1 shows the sole system of 2 coke ovens arranged next to one another as well as the gas streams

FIG. 2 a and FIG. 2 b show the stream routes and the flame formation in the sole channels according to prior art in technology and in comparison therewith the same according to the present invention

FIG. 3 shows another top view on the sole system of 2 coke ovens arranged next to one another

FIG: 4 shows another top view on the sole system of 2 coke ovens arranged next to one another

FIG. 5 shows another front view on the sole system of 2 coke ovens arranged next to one another

FIG. 1 in a top view and front view shows 2 NR/HR ovens 1 and 2 arranged next to one another, secondary air inlets 3, secondary air outlets 4, and downcomers 5. Furthermore, one can see the secondary air channels 6 integrated in the bottom floor as well as the waste gas channel 7 as well as the inner sole channels 8 and the outer sole channels 9.

FIG. 2 a shows the stream routes and the flame formation in the sole channels according to prior art in technology. Here, the crude gas—waste gas mixture of the upper oven comes from the downcomers 5 and is burnt in flames 11 and 12 with the air from the secondary air outlets 13 in the sole channels 8 and 9.

As compared therewith, by applying the inventive method and the corresponding device shown in FIG. 2 b, individual circular flow apertures 10 are provided for which enable a backflow of waste gas, thus improving the geometry of flames 11 and 12 and achieving the inventive advantages relative to the formation of contaminants.

FIG. 3 shows an example for sole channel geometry with an individual aperture 10 to generate an internal waste gas recirculation in the lower oven.

FIG. 4 gives an example for sole channel geometry with two individual apertures 10 to generate an internal waste gas recirculation in the lower oven.

FIG. 5 gives two examples for possibilities of an external waste gas recirculation in which blowers 14 each provide for the recirculation. 

1-15. (canceled)
 16. A method to homogenize the burn-off characteristics and to reduce thermal NOx emissions from a carbonization plant of the Non-Recovery Process or Heat Recovery Process, comprising a multitude of ovens, each oven having a coking chamber and comprising an oven space bordered by doors and side walls for a coal charge or a compacted coal cake and comprising a void space located thereabove, discharge devices for waste gas from the void space, feeder devices for supply of fresh air into the void space, a system of sole channels to guide waste gas and/or secondary feed air, said system being integrated at least partly into the bottom floor under the oven space, comprising recirculating waste gas generated in the oven to the combustion process of the oven upstream of the oven chamber, to the downcomers, or to the sole channel system in the lower oven, or to more than one of these.
 17. The method of claim 16, wherein the recirculation of the waste gas generated in the oven and conducted out of the combustion chamber is returned to the oven chamber, the downcomers, or the sole channel system in the lower oven by withdrawal from an external channel system of the oven and via a blower within the oven.
 18. The method of claim 16, wherein the waste gas is returned to the sole channels located upstream via apertures or channels prior to final evacuation from the oven in the sole channel.
 19. The method of claim 18, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished via an aperture in the sole channel partition wall between the sole channels.
 20. The method of claim 18, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished via several apertures in the sole channel partition wall between the sole channels.
 21. The method of claim 18, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished via one or several aperture(s) in the sole channel partition wall between the sole channels and that the adjustment of the quantity of recirculated waste gas is accomplished via sliding bricks, nozzles, or Venturi facilities.
 22. The method of claim 16, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished outside the oven.
 23. The method of claim 22, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished by means of a blower into sole channels located upstream.
 24. The method of claim 22, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished by means of a blower into the downcomers.
 25. The method of claim 22, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished by means of a blower into the primary air apertures of the oven door.
 26. The method of claim 22, wherein the recirculation of the waste gas generated in the oven and conducted out of the coking chamber is accomplished by means of a blower into the primary air apertures of the oven top.
 27. A method to reduce the operating time of a coke oven required for the complete carbonization of a coal charge, wherein individual flames in a sole channel of the coke oven are prolonged and homogenization of the burn-off characteristics of the coking gas is improved to increase the economic efficiency of the method, comprising employing the method of claim
 1. 28. A carbonization plant of the Non-Recovery Process or Heat Recovery Process for the production of coke from coal, comprising a multitude of ovens, each oven having a coking chamber comprising an oven space bordered by doors and side walls for a coal charge or a compacted coal cake and comprising a void space located thereabove, discharge devices for waste gas from the void space, feeder devices for supply of fresh air into the void space, a system of sole channels to guide waste gas and/or secondary feed air, said system being integrated at least partly into the bottom floor under the oven space, wherein waste gas generated in the oven is recirculated to the combustion process of the oven upstream of the oven chamber, to the downcomers, or to the sole channel system in the lower oven, or to more than one of these, wherein one or several aperture(s) is/are provided in the sole channel partition wall between the sole channels.
 29. The carbonization plant of claim 28, wherein apertures in a sole channel partition wall between the sole channels are closable or the waste gas quantity are adjustable by sliding bricks, nozzles or Venturi facilities.
 30. The carbonization plant of claim 28, wherein a blower is provided and connected in such a manner that waste gases conducted out of the coking chamber are conveyed into at least one of the sole channels located upstream, into the downcomers or into the primary air apertures of the oven door or oven top. 